1. Field of the Invention
The present invention relates generally to an apparatus and method for audio speakers. Particularly, this invention relates to a micro-speaker and a method of assembling micro-speakers using a liquid suspension mechanism.
2. Description of the Related Art
Conventional speakers commonly comprise a magnet assembly, and a non-magnetic, annular frame extending from the magnet assembly to support the larger end of a cone-shaped diaphragm. The smaller end of the diaphragm cone is attached to a voice coil that extends into an annular magnetic gap provided in the magnet assembly. In order to accurately locate and suspend the voice coil within the magnetic gap, the voice coil is typically attached to the surrounding frame by a corrugated annular suspension.
The voice coil is designed to oscillate axially without experiencing other types of motion such as rotation, moving obliquely to the axial direction, or moving in different directions, at different points, in the oscillation stroke. Should the voice coil scrape on the magnetic gap surfaces, the coil will experience premature failure. One solution is the use of a low volatile, oil-based, magnetic liquid suspension mechanism for locating and suspending the voice coil within the magnetic gap. The oil-based magnetic particle colloid is adhered to the voice coil and to the magnetic gap surfaces since the microscopic magnetic particles are magnetically attracted to the gap surfaces by reason of the permanent magnetic field established across the magnetic gap. The microscopic, i.e. approximately 0.01 micrometers, magnetic particles hold the liquid phase of the colloid in the magnetic gap.
The use of low volatile, oil-based, magnetic fluid in the magnetic gap, however, is not without problems. One problem is the tendency for the liquid to be blown out or drawn out of the magnetic gap during operation, thereby depleting the quantity of liquid in contact with the voice coil. This phenomenon is due to the oscillatory motions of the voice coil, which produces momentary pressure changes in the atmosphere near the end of the pole pieces and in the annular chamber surrounding the pole piece. The use of pressure compensating channels or passageways have been used to prevent this potential blow out problem. Another problem is the added cost of using specially-formulated, low volatile, oil-based magnetic fluids as the locating and suspending mechanism.
Speaker manufacturers have constantly attempted to reduce the size of loudspeakers for use in miniaturized devices such as headphones, hearing aids, cellular phones, etc. U.S. Pat. No. 5,243,662 (1993, Sogn et al.) is one example of these miniature or micro-speaker devices. It discloses a miniaturized electrodynamic sound generator having a diaphragm, a permanent magnet with pole pieces, a magnet yoke, and a coil. The coil is attached to near the margins of the diaphragm and, on the outside of the yoke, the diaphragm is bent down and attached to the outer wall of the yoke.
U.S. Pat. No. 4,742,887 (1988, Yamagishi) discloses an earphone having a housing containing a driver unit. The driver unit includes a magnetic circuit formed by a magnetic plate, a yoke and a magnet, and a vibration system formed of a diaphragm and a voice coil that is accommodated in a gap between the yoke and the magnet. The driver unit extends across the housing adjacent a sound generation opening at the front of the housing so as to divide the interior of the housing into a front cavity and a back cavity.
U.S. Pat. No. 4,320,263 (1982, Thiele) discloses a dynamic electroacoustic transducer having a magnetic pole case defining a magnetic air gap, a coil movably mounted in the air gap and spaced from the magnetic pole case with magnetic liquid extending between the coil and the case in the air gap. A diaphragm is connected to the coil and attached peripherally to the magnetic pole casing so that airtight spaces are defined above and below the coil, which are in communication with each other. The airtight sealing of the spaces prevents the liquid portions of the magnetic liquid from evaporating, which would result in deterioration of the characteristics of the dynamic electroacoustic transducer.
U.S. Pat. No. 5,335,287 (1994, Athanas) discloses a loudspeaker with an oil-based magnetic fluid suspension for the voice coil, instead of the corrugated disk suspension that is conventionally used. Specially designed vent passages are formed in the magnet assembly to prevent internal pressure build-ups, or sub-atmospheric conditions, that could cause the magnetic fluid to be blown out of the magnetic gap.
Due to market-driven cost constraints, manufacturers of micro-speakers for use in cellular phones and other widespread, consumer electronics have designed micro-speakers that do not use the corrugated mechanism for centering and locating the voice coil in the magnetic gap. A magnetic fluid mechanism for centering and locating the voice coil in the magnetic gap of a micro-speaker is also not used because the magnetic fluid reduces, i.e. dampens, the sound pressure too much in these small-sized speakers. Consequently, the voice coil is centered and suspended in the magnetic gap without the use of these two particular centering mechanisms.
The size of the micro-speakers also creates a problem for manufacturers during the assembly process. Currently, manufacturers of micro-speakers experience a micro-speaker rejection rate that is relatively high. One of the reasons for such a rejection rate is that the assembly process is a manually intensive process. The main causes of the failure is breaking of the wire, which has a typical diameter of about 0.008 in. (0.2 mm) to about 0.013 in. (0.33 mm), that attaches to the monolithic coil, deformation of the magnetic pole piece as it is an extremely thin metal plate, and touching of the wire to the yoke when the coil, which is attached to the diaphragm, is inserted into the magnetic gap of the speaker and fixed in place.
Therefore, what is needed is an assembly method that allows the voice coil to be easily centered and suspended during the manufacturing process. What is also needed is an assembly method that reduces the manufacturing process rejection rate. What is further needed is an assembly method that is inexpensive to use and whose cost is more than offset by the reduction in the failure rate during micro-speaker production.
It is an object of the present invention to provide a micro-speaker and a method of manufacturing micro-speakers that is inexpensive to implement. It is another object of the present invention to provide a method of manufacturing micro-speakers that locates and centers the voice coil in the magnetic gap during the assembly of the diaphragm/voice coil assembly to the yoke assembly. It is still another object of the present invention to provide a method of manufacturing micro-speakers that reduces the failure rate of micro-speakers during micro-speaker production.
The present invention achieves these and other objectives by providing a simple method and mechanism for locating and centering the voice coil of a micro-speaker into the magnetic gap of the driver unit of the micro-speaker during the assembly process. The method includes the step of adding a predetermined amount of a volatile magnetic fluid to the magnetic gap before inserting the voice coil into the magnetic gap. The volatile magnetic fluid locates and centers the voice coil in the magnetic gap during the assembly process. Once the voice coil and diaphragm are fixed to the support structure of the micro-speaker in such a way that evaporation of the magnetic fluid is permitted, the magnetic fluid is evaporated leaving an air gap between the voice coil and the magnetic pole pieces of the micro-speaker.
Unlike oil-based magnetic fluids that typically use a low volatile carrier liquid such as a hydrocarbon oil, the volatile magnetic fluid of the present invention is one having a relatively volatile carrier base liquid. The volatile carrier liquid typically is a volatile liquid that is capable of undergoing evaporation at room temperature or at elevated temperatures below those required for oil-based carrier liquids. Examples of volatile liquids are water and aliphatic hydrocarbon solvents such as octane, heptane and hexane. Generally, the saturation magnetization is as low as possible for use as a voice coil centering mechanism for a given speaker configuration so as not to form a thick residual layer of magnetic particles on the voice coil or the pole pieces.
The method of the present invention involves obtaining a volatile magnetic fluid and adding a predetermined amount of the volatile magnetic fluid to the magnetic gap of the micro-speaker. The volatile magnetic fluid may be added using a dispenser or by dipping a solid needle rod or a hollow rod (i.e. capillary tube) into the magnetic fluid and locating the solid rod, the hollow rod or the dispenser close to the magnetic gap. The wetting ability of the ferrofluid and the magnetic force field of the driver unit cause the volatile magnetic fluid to fill the magnetic gap of the micro-speaker. The voice coil of the diaphragm/voice coil assembly is then positioned over the centrally-located yoke, i.e. pole piece, and the voice coil is inserted into the magnetic gap. The volatile magnetic fluid will become disposed around the voice coil causing the voice coil to be located and centered within the magnetic gap. The diaphragm/voice coil assembly can then be secured into position. Once secured, the volatile magnetic fluid is evaporated leaving the voice coil suspended within the magnetic gap.